The present invention relates to a method of controlling an enamelling oven, and more particularly a method of controlling an enamelling oven in which a controlled variable such as an oven temperature is a multivariable.
There has heretofore been known a thermal gas circulating enamelling oven for fabricating insulated electric wires of copper or aluminum coated with baked insulating paint for use in electric motors and the like. As shown in FIG. 1 of the accompanying drawings, such a known enamelling oven includes a wire travelling path 1 through which a wire 4 runs from a wire inlet 2 toward a wire outlet 3. While the wire 4 is travelling through the path 1, a solvent of varnish to be baked such as ester or formalin resin which has been coated on the wire in advance is heated to a high temperature by a gas supplied through a damper 5 and combusted by a catalyst 6, the combusted high-temperature gas being circulated by a blower 7 in the directions of the arrows.
The damper 5 serves to adjust the amount of air 8 supplied and hence the amount of gas 9 such as butane supplied dependent on the amount of air 8 as controlled by the damper 5. The enamelling oven also includes other dampers. More specifically, a damper 10 is mounted on an upper portion of the oven for regulating the amount of air discharged from the oven. A damper 11 defines a pressure chamber for sealing hot air from the oven. Dampers 12, 13 cooperate with each other in feeding hot air directly into an upper portion of a vaporizing zone EZ. A damper 14 serves to divide hot air into flows directed to a curing zone CZ and the vaporizing zone EZ. Through the control of the damper 14, the amounts of hot air supplied into the curing and vaporizing zones are varied to adjust the curing of the material baked on the wire.
The degrees of opening of the dampers 5, 10, 11, 12, 14 can be adjusted to provide optimum thermal distribution in the vaporizing zone EZ and the curing zone CZ for thereby determining the curing of the varnish coated on the wire.
The thermal distribution in the vaporizing and curing zones are measured by various temperature detecting points in the oven. Such temperature detecting points include a point 16 for detecting the temperature of hot air prior to division thereof into the curing and vaporizing zones, a point 17 located where the hot air from the oven and cool air from the damper 11 are joined, a point 18 located centrally in the curing zone, a point 19 located in an upper portion of the vaporizing zone, and a point 20 located in a lower portion of the vaporizing zone.
A single enamelling oven normally enamels a variety of electric wires having a diameter ranging from 0.6 to 2 mm and travelling at various speeds dependent on the diameter of the wires. A pair of such enamelling ovens are operated at the same time each for baking ten sets of seventy electric wires. During enamelling operation, the enamelling ovens are thermally affected from each other, and the amounts of gas normally supplied to the ovens should be varied dependent on the wire diameter and the kind of varnish used.
For controlling the oven temperature under such circumstances, it has been proposed and practiced to utilize PI controllers respectively for controlling the temperature at the point 16 with the damper 5, the temperature at the point 17 with the damper 11, and the temperature at the point 20 with the damper 10. With such a prior practice, the temperatures at the points 16, 17 can be controlled at desired levels, but the temperatures at the points 18, 19, 20 which are important controlled variables in the embodiment of the present invention cannot be regulated for desired temperature distributions or gradients. The difficulty arises out of the fact that when one of the dampers is adjusted to control the temperature at a corresponding one of the measuring points to reach a desired temperature, the temperatures at the other measuring points are also caused to change.
Stated otherwise, a multiplicity of measured variables (the temperatures at the points 16, 17, 18, 19, 20) including a multiplicity of controlled variables (the temperatures at the points 17, 18, 19, 20) are variable when any one of multiple manipulation variables (the degrees of opening of the dampers 5, 10, 11, 12, 14) is adjusted. It has heretofore been impossible in such multiple-variable control to simultaneously and automatically control the manipulation variables to bring the controlled variables to desired levels.
With the multiple controlled and manipulation variables being thus correlated, the foregoing process of controlling pairs of controlled and manipulation variables has rendered the controlled variables less stable and responsive.